With a view to further improving corrosion resistance of a zinciferous electroplated steel sheet, trials are actively made to improve corrosion resistance of its zinciferous plating layer comprising zinc or a zinc alloy by uniformly dispersing silica particles into the plating layer. It is not however easy to cause uniform dispersion of the silica particles into the zinciferous plating layer. The reason is that the silica particles are negatively charged in a zinciferous acidic electroplating solution and have a tendency of hardly precipitating onto the surface of a steel sheet serving as a cathode.
As a method for solving the above-mentioned problem and manufacturing a zinc-silica composite electroplated steel sheet excellent in corrosion resistance, having, on at least one surface thereof, a zinciferous plating layer in which silica particles are uniformly dispersed, the following method is proposed:
A method for manufacturing a zinc-silica composite electroplated steel sheet, disclosed in Japanese Patent Provisional Publication No. 63-199,899 dated Aug. 18, 1988, which comprises the steps of: electroplating a steel sheet in a zinciferous acidic electroplating solution having a pH value within a range of from 1 to 4.5, which contains silica particles in an amount within a range of from 0.5 to 100 g per liter of the electroplating solution and nitric acid ions in an amount within a range of from 100 to 3,000 ppm, to form, on at least one surface of said steel sheet, a zinciferous plating layer in which silica particles are uniformly dispersed (hereinafter referred to as the "Prior Art").
According to the above-mentioned Prior Art, it is possible to manufacture a zinc-silica composite electroplated steel sheet excellent in corrosion resistance, having, on at least one surface thereof, a zinciferous plating layer in which silica particles are uniformly dispersed. As in the Prior Art, a zinciferous plating layer in which silica particles are uniformly dispersed can be formed on at least one surface of a steel sheet by electroplating the steel sheet in a zinciferous acidic electroplating solution containing silica particles and nitric acid ions, and the reason of this is estimated to be as follows:
When the steel sheet is electroplated in the zinciferous acidic electroplating solution containing silica particles and nitric acid ions, reactions as shown in the following equations (1) to (3) take place: EQU Zn.sup.2+ +2OH.sup.- .fwdarw.Zn(OH).sub.2 ( 1) EQU Zn(OH).sub.2 +2e.sup.- .fwdarw.Zn+2OH.sup.- ( 2) EQU NO.sub.3.sup.- +9H.sup.+ +8e.sup.- .fwdarw.NH.sub.3 +3H.sub.2 O(3)
The reduction reactions of zinc ions (Zn.sup.2+) according to the equations (1) and (2) above cause the increase to 5.6 in the pH value of the zinciferous acidic electroplating solution on the interface of the cathode, i.e., the steel sheet, and the reduction reaction of nitric acid ions (NO.sub.3.sup.-) according to the equation (3) above further increases the above-mentioned pH value to over 5.6. This increase in the pH value of the electroplating solution on the interface of the cathode causes the silica particles to be absorbed by the zinc ions. This makes it easier for the silica particles, together with zinc, to be precipitated on the surface of the steel sheet as the cathode, thus increasing the rate of precipitation thereof. It is thus possible to form, on at least one surface of a steel sheet, a zinciferous plating layer excellent in corrosion resistance, in which the silica particles are uniformly dispersed.
However, the above-mentioned Prior Art has the following problems: As described above, the pH value of the electroplating solution on the interface of the cathode, i.e., the steel sheet increases to over 5.6 as a result of the reduction reaction of the nitric acid ions (NO.sub.3.sup.-) contained in the zinciferous acidic electroplating solution. The resultant increase in the rate of precipitation of the silica particles improves corrosion resistance of the zinciferous plating layer. However, when the rate of precipitation of the silica particles into the zinciferous plating layer increases excessively, workability of the zinc-silica composite electroplated steel sheet is degraded. The rate of precipitation of the silica particles, i.e., the content of the silica particles in the zinciferous plating layer, which can improve corrosion resistance without degrading workability, is within a range of from 0.2 to 15.0 wt. % relative to the zinciferous plating layer.
The content of the nitric acid ions in the zinciferous acidic electroplating solution sensitively affects the pH value of the electroplating solution on the interface of the cathode. When the pH value of the electroplating solution on the interface of the cathode decreases to 5.6 or under, the rate of precipitation of the silica particles into the zinciferous plating layer decreases to under 0.2 wt. % relative to the plating layer. When the above-mentioned pH value increases to over 12, on the other hand, the rate of precipitation of the silica particles increases to over 15.0 wt. % relative to the plating layer, thus degrading workability of the zinciferous electroplated steel sheet. Therefore, the range of the content of the nitric acid ions, which is capable of increasing the rate of precipitation of the silica particles without degrading workability, is very narrow.
Upon electroplating, it is very difficult to keep the content of the nitric acid ions in the electroplating solution within the narrow range which can increase the rate of precipitation of the silica particles without degrading workability. It is therefore very difficult to stably manufacture a zinc-silica composite electroplated steel sheet excellent in corrosion resistance and workability, having on at least one surface thereof a zinciferous plating layer in which silica particles are uniformly dispersed in an amount sufficient to improve corrosion resistance without degrading workability.
Under such circumstances, there is a strong demand for the development of a method for stably manufacturing a zinc-silica composite electroplated steel sheet excellent in corrosion resistance and workability, having on at least one surface thereof a zinciferous plating layer in which silica particles are uniformly dispersed in an amount sufficient to improve corrosion resistance without degrading workability, but such a method has not as yet been proposed.